Fouling of heat transfer surfaces of petroleum processing equipment occurs continuously during the period when petroluem or its derivatives are being processed in the equipment. The fouling is caused by the gradual buildup of a layer of high molecular weight polymeric material resulting from the thermal polymerization of unsaturated materials which are present in the petroleum. As time goes by, fouling continues with the attendant loss of heat transfer until finally the point is reached where it becomes necessary to take the equipment out of service for cleaning. Cleaning is expensive and time consuming; consequently, methods of preventing fouling, or at least significantly reducing the rate of fouling, are constantly being sought.
The most economical method of reducing the fouling rate in process heat transfer equipment is to add chemicals which inhibit fouling, called "antifoulants", to the feed stream being processed. Among the more interesting classes of chemical compounds which exhibit antifoulant activity are the phenothiazines. Their use to inhibit polymerization has been described in several patents.